Red land crabs are key to the health of the Christmas Island ecosystem, Abbott says. “The red crab is a keystone species and a gardener that eats seeds and seedlings and leaf litter and creates a very open understory [in the island's rainforest].

Baby red crabs on Christmas Island (Photo: Parks Australia)

To kill a crab, the ants spray formic acid from their abdomens and then either let an immobilized crab dehydrate to death or rip it to pieces. By 2002, super-colonies of yellow crazy ants “covered over 2,500 hectares [of rainforest on Christmas Island],” Abbott says, making life very hard for red land crabs. “On every surface in three-dimensions [was] a moving carpet of yellow ants.”

“By killing the red land crabs, [the ants] were removing a keystone species,” Abbott says. “They actually changed the entire structure and composition of the rainforest [on Christmas Island] … The seeds grew into seedlings and clogged the understory of the rainforest. The leaf litter built up and created a habitat for unwanted customers.”

Abbott was determined to figure out how this explosion of ants happened. Her hypothesis? The weather. More specifically — a dry, stressed forest. The thing is, crazy yellow ants do not live on crabs alone. Their super-colonies depend on carbohydrates from a type of bug called, Tachardina aurantiaca, or yellow scale bug. This scale-shaped parasite sucks sap from trees and then releases “a sweet secretion from their anal pore called honeydew,” which the ants eat.

And these scale bugs thrive when trees are stressed, Abbott says, because when trees are stressed “they mobilize nitrogen and other elements in their phloem that sap-sucking insects can access a little bit more readily … [elements] that are otherwise bound up in defense systems,” she says.

When the crazy ant populations exploded “there was an El Niño system over the Indian Ocean,” Abbott says. “The forest [on Christmas Island] was incredibly dry and water-stressed.”

Yellow crazy ants with scale insects (Photo: Luke S. O’Loughlin)

While an El Niño system could be the explanation for the rise of the crazy ants, Abbott isn’t completely sure if it is. She and her fellow researchers are still searching for the full story — and working on ways to re-balance the Christmas Island ecosystem.

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